1. Field of the Invention
The present invention generally relates to squib fire networks which include a built-in testing system, and a method to evaluate the operational status of, and to determine faults within, a squib fire network.
2. State of the Art
A squib fire network includes at least one squib, a squib being an explosive device that comprises resistive elements, such as bridge wires. The bridge wires can be surrounded by explosive charges. When the resistive elements are charged through the passage of current, they detonate the explosive charges which explode and create a pressure wave that can be used to operate valves, explode larger charges (for use in weapons) and move pistons, and so forth. Within a missile for example, the firing of twenty-five or more squibs can be required to deliver the missile to its intended target. For example, squibs can be used to activate batteries, deploy wings, close doors, start engines, and actuate fuel valves.
Authorities have been reluctant to test squibs installed in a device, for fear of activating the device in response to an unintended current. This is especially true for squibs used in military applications. For example, missiles can be held below a ship's deck in storage bunkers. The testing of the electronics of these missiles is quite dangerous. A broken switch could ignite the missile and cause an explosion. As such, military authorities have refrained from passing current through squibs to test their operability. In fact, this type of testing has been strictly forbidden, as closing any contacts that would normally be used to fire the squib during a detonation sequence of a missile presents too much potential danger.
For example, U.S. Pat. No. 3,892,182 describes a squib control circuit which has squib filaments connected in series with a relay winding. A small amount of current is applied to the squib filaments, which light a lamp assciated with the relay, indicating the squib is ready to fire. Although the current directly applied to the squib is normally not high enough to "fire" the squib, a transient surge or spike in the current could inadvertently fire the squib. Thus, restrictions imposed on the testing of squib fire networks have rendered this test routine inapplicable to weapons because of its susceptibility to inadvertent firings.
U.S. Pat. No. 3,619,792 discloses a self-test circuit for use in a weapons drop situation. This system reports a failure of a weapons release subsequent to a release sequence. Although partial testing of squibs is performed, there is no testing to ensure that a current through the squib will ignite the squib before the weapon is actually delivered to its target.
Typically, at most two switches are involved in closing a squib fire network to provide fire current to the squib. Two switches are normally used so that if one switch fails and allows current to the squib, the second switch will block the current and the squib will not be prematurely ignited. For example, U.S. Pat. No. 5,621,326 discloses a squib which can be used for an airbag placed in an automobile. This system has the squibs placed between an acceleration switch and a second switch on the output side of the squib. However, should either squib produce a shorted condition after its activation, then the remaining squib may be starved of current and be unable to be activated. Furthermore, this circuit is not able to selectively activate a squib. Because this circuit increases the possibility of switch failure and inadvertent squib fire, it is inappropriate for use in all military applications.
The restrictions imposed on the testing of squib fire networks used in weaponry have left the reliability of missiles employing these devices in question until the missiles are actually deployed. Accordingly, there is a need for a squib fire network having a cost effective, built-in testing system to detect faults with a high level of accuracy without the potential for actuating squibs of the squib fire network. If the built-in testing system could determine whether all the squibs are in proper working order prior to launch, repairs could be performed on the weapon before deploying it, thus reducing weapon malfunctions in the field, and the number of weapons that must be expended to accomplish the field objective.